U.S. patent number 4,249,224 [Application Number 06/018,337] was granted by the patent office on 1981-02-03 for surge voltage arrester with fail-safe feature.
This patent grant is currently assigned to Reliable Electric Company. Invention is credited to Bertram W. Baumbach.
United States Patent |
4,249,224 |
Baumbach |
February 3, 1981 |
Surge voltage arrester with fail-safe feature
Abstract
A surge voltage arrester utilizes for surge protection a
varistor of the zinc oxide type between a line terminal and a
ground terminal. The varistor is of a type that has a resistance
which decreases as increasing voltage is applied thereacross. With
increasing temperature the leakage current through the varistor
increases at a given voltage. At a critical temperature and voltage
condition, the varistor is subject to a thermal runaway condition
which would cause the varistor to fail expelling hot particles. To
avoid this failure condition, a fail-safe mechanism is provided
which connects the line terminal to the ground terminal by-passing
the varistor when there is an overcurrent condition on the line
that results in heating of the varistor.
Inventors: |
Baumbach; Bertram W. (Arlington
Heights, IL) |
Assignee: |
Reliable Electric Company
(Franklin Park, IL)
|
Family
ID: |
21787411 |
Appl.
No.: |
06/018,337 |
Filed: |
March 7, 1979 |
Current U.S.
Class: |
361/124; 337/28;
337/32; 337/33; 338/21; 361/127; 361/57 |
Current CPC
Class: |
H02H
9/043 (20130101); H01C 7/126 (20130101) |
Current International
Class: |
H01C
7/12 (20060101); H02H 9/04 (20060101); H02H
003/22 (); H02H 009/04 () |
Field of
Search: |
;361/124,127,118,119,54,55,57,103,104,106 ;337/28,31,32,33,34,15
;338/20,21,31 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Salce; Patrick R.
Attorney, Agent or Firm: Trexler, Wolters, Bushnell &
Fosse, Ltd.
Claims
This invention is claimed as follows:
1. A surge voltage arrester for a communications line comprising a
line terminal adapted for connection to a line to be protected, a
ground terminal, means forming a first electrical circuit between
said terminals that is substantially non-conductive for line
voltages below a predetermined value but becomes substantially
conductive to arrest surge voltages on the line above said
predetermined value, said first circuit including circuit component
means having a resistance that decreases as increasing voltage is
applied thereacross and which heats excessively in an overcurrent
condition in the protected line, said component means being further
characterized as passing increased current for a given voltage upon
increased temperature, a normally-open second electrical circuit
coupled between said terminals and including a pair of contactors
electrically connected respectively to said terminals, means
applying pressure tending to bring said contactors into engagement,
and heat-deformable means operable in opposition to said
pressure-applying means to maintain said contactors apart, said
heat-deformable means deforming sufficiently upon heating of said
component means to cause said contactors to engage so that said
second circuit closes and provides a direct conductive path between
said terminals.
2. A surge voltage arrester according to claim 1 in which said
component means comprises a varistor formed of a zinc oxide
varistor compound.
3. A surge voltage arrester according to claim 1 in which said
component means comprises a structure having opposed faces, at
least one of said faces being in thermally conductive connection
with a member that has a thermal conductivity substantially that of
copper and having a region at the peripheral part of said member,
said heat-deformable means being substantially of said region.
4. A surge voltage arrester according to claim 1 in which said
component means comprises a structure having opposed faces, and
means forming heat sinks on said faces, one of said heat sinks
transferring heat from the component means to said heat-deformable
means.
5. A surge voltage arrester according to claim 4 in which one of
said contactors is a receptacle that contains an assembly
comprising said component means and said heat sinks.
6. A surge voltage arrester according to claim 1 including an
additional line terminal, a third circuit similar to said first
circuit and between said additional terminal and said ground
terminal, the component means of the respective first and third
circuit being on opposite sides of said ground terminal.
7. A surge voltage arrester comprising a line terminal adapted for
connection to a line to be protected, a ground terminal, means
forming a first electrical circuit between said terminals that is
substantially non-conductive for line voltages below a
predetermined value but becomes substantially conductive to arrest
surge voltages on the line above said predetermined value, said
first circuit including a varistor that passes increased current
for a given voltage upon increased temperature and which is capable
of heating to a thermal runaway condition in an overcurrent
condition in the protected line, a normally-open second electrical
circuit coupled between said terminals and including a pair of
contactors electrically connected respectively to said terminals,
means applying pressure tending to bring said contactors into
engagement, and heat-deformable means operable in opposition to
said pressure-applying means to maintain said contactors apart,
said heat-deformable means being in thermal conducting relation to
said varistor, said heat-deformable means deforming sufficiently
upon heating of said varistor in said overcurrent condition to
cause said contactors to engage so that said second circuit closes
and provides a direct conductive path between said terminals.
8. A line protector comprising a line terminal and a ground
terminal, a first electrical circuit from the line terminal to the
ground terminal and having varistor means heatable in the event of
an overcurrent condition on the line, a normally open second
circuit coupled between said terminals and including heat sink
means in thermally conductive relation with said heatable varistor
means for producing one or more hot spots in said heat sink means
in said overcurrent condition, spring loaded contactor means in
said second circuit having multiple fingers tending to close said
contactor means and thereby close said normally open second
circuit, and means normally preventing said contactor means from
closing and being heat deformable at least in the region of a hot
spot such that one or more of said fingers closes said second
circuit depending upon the hot spot location.
Description
BACKGROUND OF THE INVENTION
This invention relates to surge voltage arresters of the type
primarily used for protecting telephone and like communication
lines from overvoltage or overcurrent surges. The principles of the
present invention are applicable to station protectors as well as
to central office protectors. A typical central office protector is
shown in U.S. Pat. No. 3,818,271, granted June 18, 1974 to
Baumbach.
The use of varistors of the zinc oxide compound type in surge
voltage arresters is broadly known as by reference to U.S. Pat. No.
4,092,694 to Stetson, granted May 30, 1978. Varistors of this type
are non-linear voltage dependent resistances in which the
resistance decreases as increasing voltage is applied across the
varistor. Such varistors are also sensitive to heating. With
increasing temperatures, the leakage current across the varistor
increases at a given voltage. The increased leakage current further
raises the temperature with the result that at a critical
temperature and voltage condition, the varistor becomes subject to
a thermal runaway condition and fails due to passing
ever-increasing current. Upon failure generally a hole will develop
in the varistor and hot particles will be expelled. Such a
condition is obviously unsuitable for use in a central office line
protector or in the proximity of other equipment because damage to
such equipment and to personnel becomes possible.
The thermal runaway condition can be opposed by improving the heat
transfer between the varistor and its supporting housing. However,
the use of some form of improved thermal coupling may not be
adequate to prevent thermal runaway under high surge voltage
conditions of long duration, particularly if the surge voltage
arrester is to be produced at reasonable costs.
OBJECTS AND SUMMARY OF THE INVENTION
An object of this invention is to provide a surge voltage arrester
which utilizes a varistor of the type stated and can be embodied
into a device with a fail-safe mechanism whereby the arrester unit
may be utilized as a station or central office protector, or in
other applications proximate to other telephone equipment.
A further object of this invention is to provide a surge voltage
arrester of the type stated in which it is possible to provide for
protection of currents up to about 450 amps utilizing 22 gauge AWG
wire in the incoming line cable while preventing destruction of the
varistor and the expulsion of hot particles therefrom at high surge
currents.
A still further object of this invention is to provide a surge
voltage arrester of the type stated that utilizes the advantages of
a varistor in that the varistor fails "closed," that is by shorting
to ground, in contrast to many gas tubes which fail open with a
loss of gas.
In carrying out the foregoing objects the surge voltage arrester
comprises a line terminal adapted for connection to a line to be
protected, a ground terminal, means forming an electrical first
circuit between said terminals that is substantially non-conducting
for line voltages below a predetermined value but becomes
substantially conducting to arrest surge voltages on the line above
said predetermined value, said first circuit including circuit
component means having a resistance that decreases as increasing
voltage is applied thereacross and which heats excessively in an
overcurrent condition in a protected line, a normally open second
electrical circuit between said terminals and including a pair of
contactors electrically connected respectively to said terminals,
means applying pressure tending to bring said contactors into
engagement, and heat-deformable means operable in opposition to
said pressure-applying means to maintain said contactors apart,
said heat-deformable means deforming sufficiently upon excessive
heating of said component means to cause said contactors to engage
so that said second circuit closes and provides a direct conductive
path between said terminals.
The varistor is formed of a zinc oxide ceramic varistor compound
and is further characterized in that it passes increasing current
for a given voltage upon increasing temperature. Also, the varistor
is typically a disc-shaped component having opposed faces and
wherein this invention uses copper plates disposed against those
opposite surfaces to provide heat sinks to transfer heat away from
the varistor. It has been found that this heat tends to migrate
toward the perimeters of the copper plates and for that reason the
heat deformable or fusible means, which may be of low melting point
solder, is disposed in the peripheral region of one of the copper
plates.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a top plan view of a surge voltage arrester constructed
in accordance with and embodying the present invention;
FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;
FIG. 3 is an enlarged portion of FIG. 2;
FIG. 4 is a top plan view of a modified form of a surge voltage
arrester embodying the invention; and
FIG. 5 is a sectional view taken along line 5--5 of FIG. 4.
DETAILED DESCRIPTION
Referring now in more detail to the drawing, particularly to FIGS.
1-3, there is shown a surge voltage arrester comprising an
insulator 2 which serves as a mounting or support for the arrester.
Secured to the insulator 2 at spaced regions therealong is a
metallic line terminal 4 and a metallic ground terminal 6. The line
terminal 4 and the ground terminal 6 are suitably secured
respectively as by screws and nuts 8, 10 to the insulator 2. The
line or central office equipment to be protected is wire-connected
to the screw 8 whereas the screw 10 is connected to ground in any
suitable fashion.
In the form of the invention shown, the ground terminal 6 is in the
form of an angle-shaped bracket that has a part that projects away
from the insulator 2 for supporting an annular brass cup or shell
12. The base of cup 12 rests upon the ground terminal 6. The
annular sidewall 14 of the cup 12 projects toward the line terminal
4.
Disposed within the cup 12 is an assemblage that includes a
varistor 16 of the zinc oxide ceramic type. Typically, the varistor
may be of a known type designated as V250LA40 and manufactured by
the General Electric Company, Pittsfield, Mass., U.S.A. The
varistor 16 is approximately 0.120 inches thick with metalized
opposed faces 18, 18 that serve as contacts. The varistor 16 may be
a single unitary circuit component, but it should be recognized
that such a component may consist of a plurality of face-to-face or
"stacked" varistors in series. In any event, the faces 18, 18 of
the varistor 16 have solder films 20, 20 onto which are soldered
copper discs or plates 22, 24. The copper plate 24 abuts the bottom
surface of the shell 12 and is slightly smaller in diameter than
the internal diameter of the shell sidewall 14 so as to fit loosely
within the shell 12. The other copper plate 22 is approximately the
same diameter as that of the varistor 16.
The copper discs 22, 24 serve as heat sinks to delay the tendency
to destruction that takes place in the varistor component 16 during
high leakage current conditions through the varistor. It has also
been found that the copper discs 22, 24 tend to force hot spots
formed in the varistor during high current overloads to move toward
the peripheral parts of the copper plates 22, 24. A heat-deformable
ring 26 of low melting point solder rests on the copper disc 22 and
is of a diameter such that the ring 26 is adjacent to the
peripheral part of the copper disc 22. Thus, under high current
overloads against which protection is sought, the solder ring 26
will melt in the region of the hot spots allowing one or more of
the hereinafter described multiple fingers to provide a conductive
path to ground.
Disposed against the solder ring 26 is a spider 28 of spring brass
or the like. The spider 28 has multiple spring fingers 30 which
radiate from a center part 32. The spider 28 is normally flat but
may be spring biased to the condition shown in FIG. 2 under axial
pressure against the center 32. This axial pressure may be applied
through a rod 34 that is soldered or is otherwise joined to an end
of the line terminal 4. Thus, with the varistor 16 and copper
plates 22, 24 placed within the shell 12, the solder ring 26 may be
positioned on the copper plate 22. The spider 28 may then be
positioned over the open end of the shell until the fingers 30 of
the spider seat on the solder ring 26. When the line terminal 4 is
attached to the insulator 2 by the screw 8, the rod 34 will be in
such position as to apply axial force against the spider center 32
depressing it inwardly toward the copper plate 22 and pressing the
fingers 30 firmly against the solder ring 26. The tip portions of
the finger will angle upwardly and will be spaced from the rim of
the cup sidewall 14 as shown in FIG. 3 and in full lines in FIG. 2.
Thus, the fail-safe circuit will be open during normal conditions
of operation.
A high surge voltage of short duration and low current appearing at
the line terminal 4 will be conducted to ground through a circuit
including the varistor 16 and the ground terminal 24. Under such
conditions the arrester is self-restoring for further protection.
However, the device is also a line protector under abnormally high
current overloads. Thus, the heat from the varistor 16 will be
transmitted through the copper plate 22 to the solder ring 26
causing the solder ring 26 to deform or melt at least in the
regions of the hot spots on the plate 22. All of the spring fingers
30 are then no longer opposed by the solder 26. Thus, one or more
of the spring fingers 30 and the rim of the sidewall 14 will
contact each other, as shown in broken lines in FIG. 2, to provide
a direct conductive circuit or path from the line terminal 4 to the
ground terminal 6. Which of the fingers 30 provides the conductive
path to ground may depend upon the location of the hot spot or
spots. The heat transferred by the copper discs 22, 24 away from
the varistor 16 helps control thermal runaway. Also, the fact that
the solder ring 26 is at the peripheral region of the copper disc
22 assists in rapid melting of the solder ring 26 under high
current conditions. This actuates the fail-safe circuit to ground
the line terminal 6 before a thermal runaway has caused eruption of
the varistor or the expulsion of hot particles therefrom.
FIGS. 4 and 5 show an arrangement in which two varistor type
assemblies may provide an arrester that results in protection from
each of two line terminals 40, 40. The parts in FIGS. 4 and 5 that
are similar to the parts in FIGS. 1-3 are designated with like
reference numerals. One terminal may be connected to each side (tip
and ring) of the line. However, the copper plate 24a may be of
substantially the same diameter as the remainder of the varistor
assembly, which is otherwise the same as in FIGS. 1-3. The ground
contactor 60 may include a tab or terminal 62 at which the
contactor 40 may be grounded. A varistor 16 and its assembled
copper plates is disposed on each side of the ground contactor 60.
Each resilient brass spider 28a, 28a has its fingers 30a, 30a
somewhat longer than the fingers 30 of FIGS. 1-3. These spring
loaded fingers 30a are bent to project toward the contactor 60 with
the ends of the fingers 30a normally held spaced therefrom by the
solder rings 26, 26 as shown in FIG. 5. However, upon deforming or
melting of one of the solder rings 26, one or more of the
associated fingers 30a will move toward the contactor 60 and into
engagement with contactor 60 to ground the associated line.
By way of example but not of limitation, the copper discs 22, 24
may be 0.031 inches thick, the spring brass spider 28 may be 0.016
inches thick and the brass cup or shell 12 may be 0.040 inches
thick. A nominal outside diameter for the brass cup is about 1
inch. The solder films 20, 20 embody solder that melts at
approximately 360.degree. F. On the other hand, the solder ring 26
may utilize solder that melts at 204.degree. F. and may be in the
form of a wire whose diameter is approximately 0.061 inches.
* * * * *